this allowed expanding development without the need for large flood plains. kharaghani: the los angeles river is approximately 51 miles. concrete reduces the size of the river that you need to carry the water because it speeds up the flow of water. if you'd like to remove all the concrete and to have natural system to carry the water to protect you from flooding, you need to have almost one mile on each side of the river set aside for transport of rain. so in order for the city to have use of those lands around the river, concrete made it possible to have minimum land so the water can be carried to the ocean in the fastest possible way. but, unfortunately, because of population growth and poor housekeeping, people are putting a lot of pollutants on the street and the streets are the openings to our stormwater sewer system. in an average year, 4,000 to 5,000 tons of trash ends up in long beach. man: we've created a system to deal with flooding but inadvertently created a pathway for pollution to get into our waterways. narrator: los angeles county is beginning